Radio communication with mobile users in moving vehicles, for example, is typically provided via fixed base stations or cell sites. Each cell site includes one or more antennas arranged for transmission of signals to and reception of relatively low power signals from mobile user equipment presently within a limited geographical area around the cell site. To provide the desired antenna coverage, the area around the cell site may be divided into sectors. Four sectors each of 90 degrees in azimuth coverage, or three 120 degree sectors, thus provide 360 degrees azimuth coverage around a cell site. The cell terminology reflects the necessity of providing a pattern of adjacent cells, each with its own cell site antenna installation, in order to provide mobile communications coverage over a geographic area or region larger than the limited area covered by a single such installation.
Considering one of the 120 degree sectors referred to, it is relatively straightforward to design an antenna system providing 120 degree sector coverage. The antenna system will be effective to transmit signals to any user within the desired sector coverage area. If signals are too weak to reliably reach users at the outermost edge of the sector coverage area, transmitter power can be specified at a higher level. However, for signals to be received from a user at the sector edge, user transmissions are normally subject to limitations in power transmitted from the mobile equipment. If the user's transmitted power is inadequate to achieve reliable reception from the outer portions of the sector coverage area, the overall size of the cell and the gain of the receive antenna at the fixed cell site become limiting considerations. A number of interrelated factors, including expected user population and available broadcast frequency spectrum, may also be involved in determining the optimum size of the coverage area of each cell site. It will be apparent, however, that with smaller coverage areas additional cell site installations will be necessary to provide continuous coverage over a geographical region. Additional cell site installations require the purchase, installation and maintenance of more equipment as well as increased requirements and costs of site acquisition, interconnection facilities and system support.
Use of higher gain receive antennas at each cell site would permit reliable reception of user signals at greater distances. However, for a given type of antenna, gain is directly related to beamwidth and an antenna providing coverage over a 120 degree azimuth sector typically provides relatively low gain performance. High gain may be achievable by use of larger antennas, however size and cost may become limiting factors. Higher gain is also possible by use of narrow beamwidth antennas providing coverage of only a portion of a sector. Use of antenna systems providing sector coverage by provision of a plurality of narrow beams could be arranged to provide higher gain than available by use of a single wide beam sector antenna. However, for a multi-beam antenna the antenna pattern gain provided at beam crossovers between adjacent narrow beams will be significantly lower than the peak gain provided along the beam centerline of each narrow beam. For full sector coverage, the improvement in gain achieved by replacement of a single wide beam sector antennas with antennas providing two or more narrower beams is, therefore, limited by the effective gain provided at the crossover between adjacent beams.
Objects of the present invention, therefore, are to provide new and improved multi-beam receive antenna systems and such systems which provide one or more cost or operational advantages over prior mobile communication or other types of antenna systems.